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The control unit, operating directly from the power source, impresses a small voltage on the sensing elements.

When an overheat condition occurs at any point along the element length; the resistance of the eutectic salt within the sensing element drops sharply; causing current to flow between the outer sheath and the center conductor. This current flow is sensed by the control unit, which produces a signal to actuate the output relay.

When the fire has been extinguished or the critical temperature lowered, the Fenwal CFD®  System automatically returns to standby alert, ready to detect any subsequent fire or overheat condition.
 
The Fenwal CFD®  System maybe wired to employ a "loop" circuit. In this case, should an open circuit occur, the system will still signal fire or overheat. If multiple open circuits occur, only that section between breaks becomes inoperative.